Topic: Bored stock carbs on 350 engine

[andyman]

Hey there!
Is there somebody who has installed bored stock carbs on a 350 Four engine?

Is there much more power?

Greets Andy

[gschuld]

Are you referring to bored stock carbs on an otherwise dead stock cb350 four engine?

If so, I suspect you would be disappointed. Factory carbs generally flow plenty for the original engine specs.  Overcarbing typically causes negative effects such as signal loss hurting bottom end.  On a dead stock cb750 engine, anything more than the factory carbs IMHOis not making any real difference.

On a performance engine, it’s common to want carbs to flow 5-10% more cfm than the cylinder head can flow at peak intake lift.  More will not help anything.

On a modified 350-4 engine with greater airflow demands, stock carbs may come up short.

George

[andyman]

Hi George!
Thanks for answering!

My 350 engine has installed a WebCam Satge 1 Camshaft and a mild ported cylinderhead. Pistons are original 347ccm, exhaust original with open megaphones 4into4.
My carbs are without choke.

Are the bored carbs to big for the 350 engine? I know the carbs only for 400 Four.

Greets Andy

[andyman]

Nobody here who has installed the bored carbs on a 350???

[gschuld]

Bored carbs are pretty rare.  Most wanting more carb but an aftermarket set of carbs. 

Do you have a set or would be looking to have a set bored out?

George

[gschuld]

https://s3.amazonaws.com/sohc4-articles/CB400/1977_CW_Yoshimas_Business_Card.pdf

FWIW:

George

[Don R]

 I looked up the 350 and 400 carbs, it seems they are different but I'm not sure in what way. I'd think more CC's would be the next step up.
 I watched engine masters today and they tested a V8 with three different carbs, vacuum secondary, double pumper with choke and a full race double pumper with smoothed inlets and no chokes. No matter which carbs they installed the total cfm of engine airflow stayed the same, their theory is the engine is just an air pump. The difference in the quality of the air flow and the way it affected the fuel mix did change the horsepower though.
  The race carb made more power at higher rpm's and as expected the vacuum secondary street carb made more power at lower rpm's. This is due to the better fuel mix of more air velocity when the secondaries were closed.
  So, more or better airflow can make power if the carb metering is happy but there is a tipping point where the air goes slower and doesn't pull as much vacuum past the jets.
  Your mileage may vary, and of course, I may be proven wrong at any minute.

[andyman]

My question is because the bored carbs could be to big for the 350 engine, Stage 1 camshaft, ported head, open 4into4.

[Bodi]

From comparing 350 and 400 carbs (without measuring bores) it looks like the bodies are identical so the setting number is due to different brass parts. The bored 400 carbs might be worthwhile on a 466 engine but probably pointless on a 347 in my opinion. I have a set of 400 carbs I've considered having bored, but the boring cost and the work to disassemble and reassemble... then the ordeal of tuning them, has kept them on the shelf for probably 20 years.

[Leino]

What size are the stock carbs?

[HondaMan]

In my experience....adding a properly-done step-bore (either 0.25mm or 0.5mm oversize) will add more HP to your 350F than any type of carbs you might add. As with all of these SOHC4 bikes, this engine normally picks up 10% more HP from a properly-done step-bore because of the warping of the cylinders, especially #1 and #4, which is normal in these engines. The #1 issue to watch during such a rebuild is: make DEAD SURE the piston-bore clearance is LESS than 0.001" (I use 0.0004" to 0.0006" max), as most machine shops will not do this tight tolerance unless you either insist on it, sign a waiver, or otherwise make them feel real uncomfortable. This means they must bore it so that the pistons will not fit into the holes until after it has been finish-honed, and the proper such finish honing removes about 0.0008", so this means they must bore to 0.0002" LESS than the piston skirt diameter prior to honing.

I rebuilt my own 750 with 0.0006" piston clearance, and it runs better than it EVER did.